1. Field of the Invention
The present invention relates to a process for the production of ignition retardant polymers, and in particular to a process whereby thermoplastic elastomer is incorporated during polymerization to mitigate the effect on impact strength of adding ignition retardant additives.
2. Description of the Prior Art
Polymers derived from styrene have been commercially applied for many years, including the "high impact" modifications in which the impact strength is improved by incorporating a minor amount of an elastomer during the polymerization. See, for example, U.S. Pat. Nos. 2,694,692 and 3,485,894. The replacement of conventional elastomers by radial block copolymers leads to a breakdown of the structural morphology, as disclosed in a paper presented by Feniak to CIC/SPE Meeting, Mississauga, Ontario, on Nov. 5, 1981. It is also known to improve the flame resistance of polystyrene, including its high impact modifications, by blending with flame retardant additives, such as brominated diphenyl ether and antimony oxide, see, e.g., U.S. Pat. No. 4,000,114. Since the incorporation of such flame retardants into the polymer may lower its impact strength, a further amount of rubber is often blended with the ignition retardant polymer to compensate for that loss for impact strength.
An existing procedure for producing such ignition retardant products is through physical blending of the rubber-modified high impact polystyrene with the appropriate quantities of ignition retardant additives and further rubber. However, such a procedure inevitably involves significant blending and handling costs, and considerable improvements in operational efficiency, and hence savings in costs, would be obtained if the ignition retardant additives and the supplemental rubber could be added directly into the polymer in the course of its production. The brominated nature of most ignition retardants creates practical difficulties to their presence during the polymerization process, but the incorporation at that stage of at least part of the supplemental rubber would confer useful benefits.